The Assimilation Suite discovers systems, services, network connections, configuration and dependencies, IP and MAC addresses. This all goes into a continually updated graph-based configuration management database (CMDB). This is then compared and scored against best practices, services and servers are monitored - all with near-zero configuration - in a way that scales to hundreds of thousands of servers.
It also provides visualization tools, APIs for sending alerts to humans and other systems, and a variety of canned reports (queries) to aid in securing and managing systems, hooking into ChatOps, and creating plans for triaging your security issues

Buildbot is an open-source framework for automating software build, test, and release processes. It differs from other CI tools in that it is more of a betteries included Framework, rather than a tool, and can hence be grown to meet your own needs. One of the reasons it is used in many high profile open source projects like Chromium, Firefox, Webkit, Python, etc.

Cabot is an open source, self hosted infrastructure monitoring tool. It can monitor your services and send telephone, SMS, email or chat alerts to your team whenever anything goes down, all without writing a single line of code.

Check_MK is an extension to the Nagios monitoring system that allows creating rule-based configuration using Python and offloading work from the Nagios core to make it scale better, allowing more systems to be monitored from a single Nagios server. It comes with a set of system checks, a mod_python and JavaScript based web user interface, and a module that allows fast access to the Nagios core. On top of Nagios it also adds additional features

ConfigCat is a feature flags as a service. It gives you a web based dashboard to manage your feature flags + SDKs to integrate those feature flags into your applications. ConfigCat enables you to use the same feature flags in your mobile apps, websites and server-side applications. It offers all features for free. You get an enterprise-ready SLA with the pricing plans.

A generic integration tool built on a Message Queue, to make it easy to communicate events and triggers across various parts parts of the infrastructure. Created to ease the pain of configuring many disparate events in a CI

The Ruby Cloud API library. Provides a Ruby API for interacting and managing many cloud providers, such as spinning up new instances, terminating old ones, attaching storage, etc. Currently supports more than 20 providers, including EC2, Rackspace, Brightbox, etc.

A complete lifecycle management tool for servers, both self-hosted and in the cloud. It allows you to create and manage instances, build and deploy images, view and audit hosts, and much more! It works with Puppet or Chef, and has multiple interaction facilities like a web frontend, CLI and a RESTful API.

Honeybadger is a hosted monitoring service that tracks errors happening in your applications, aggregates them, and notifies you about them based on rules you define. Honeybadger gives you the insight you need to diagnose and fix application errors, faster.

Icinga is an enterprise-grade monitoring system. It was forked from Nagios and has added many often new features and patches, including a HTML5 UI, REST interface, dynamic tables, etc. It is also backwards compatible with Nagios, so all the existing plugins can be reused.

Nagios Log Server greatly simplifies the process of searching your log data. Set up alerts to notify you when potential threats arise, or simply query your log data to quickly audit any system. With Nagios Log Server, you get all of your log data in one location, with high availability and fail-over built right in.

netdata is a system for distributed real-time performance and health monitoring. It provides unparalleled insights, in real-time, of everything happening on the system it runs (including applications such as web and database servers), using modern interactive web dashboards.

Prevoty enables DevOps to inject security into your web applications and web services. Protection is provided at runtime from within the application itself, and stays with the app regardless of deployment method (cloud, on-premises, etc). Applications are protected against threats including XSS, SQLi, CSRF, command injection, etc. Works with Puppet, Chef, Ansible, Docker, Splunk, QRadar, ELK, and many others.

Create, deploy, and manage Cloud Native infrastructure, as real code. A single toolset for infrastructure, managed services, containers, and serverless environments on any cloud infrastructure. You get all the benefits of real programming languages; IDEs, abstractions and reuse thanks to functions, classes, and packages, debugging, testability, and more

Semaphore is a hosted continuous integration and deployment service for open source and private projects. Its cloud solution helps developers find bugs before they reach users and deliver new features faster.

A set of Ansible playbooks to configure and maintain your own personal cloud, including email, calendars, file storage, vpn, irc, etc. A nice collection of utilities as well as a good showcase of Ansible's features

A platform for event-driven automation. StackStorm allows you to integrate and automate across services and tools. It ties together your existing infrastructure and application environment so you can more easily automate that environment -- with a particular focus on taking actions in response to events.